When a radio communication terminal device is powered on, the radio communication terminal device searches for a base station and connects to the base station. In this case, the radio communication terminal device uses a frequency at which a communication carrier provides a service. The term “communication carrier” as used herein refers to a network operator, a communication service provider, a telephone company, or the like. The radio communication terminal device performs the above-described processing, even when it is outside a communication service area.
However, immediately after the radio communication terminal device is shipped from a factory or when the radio communication terminal device is used abroad, the radio communication terminal device searches for a frequency at which a desired communication carrier provides a service (the search is also called “Network Discovery”). The reason why the radio communication terminal device searches for a frequency is that it does not have frequency information of the communication carrier.
The frequency search generally involves two procedures: band search and carrier search. In the band search, frequencies at which radio frequencies arrive are searched for. The radio communication terminal device performs the band search by checking each of the frequencies included in the frequency band for radio communication. In the carrier search, a frequency used by a desired communication carrier is searched for. The radio communication terminal device performs the carrier search by checking the frequencies found in the band search. Upon finding the frequency used by the desired communication carrier, the radio communication terminal device connects to a base station by using the found frequency.
Information used for identifying the communication carrier is reported from the base station. For example, in WiMAX (Worldwide Interoperability for Microwave Access), the communication carrier identification information is contained in a control message called a “downlink channel descriptor (DCD)”. The radio communication terminal device captures a radio wave at the frequency found in the band search. Next, the radio communication terminal device receives communication carrier identification information from the base station using the found frequency. The radio communication terminal device then uses the communication carrier identification information to determine whether or not the frequency is used by the desired communication carrier.
A technology in which a radio communication terminal device searches for an only frequency at which a signal carrier wave exists is known (refer to Japanese Laid-open Patent Publication No. 2007-116561).